U.S. patent application number 16/512450 was filed with the patent office on 2019-11-07 for microstructured zno coatings for improved performance in cu(in, ga)se2 photovoltaic devices.
The applicant listed for this patent is The Government of the United States of America, as represented by the Secretary of the Navy, The Government of the United States of America, as represented by the Secretary of the Navy. Invention is credited to Robel Y. Bekele, Jesse A. Frantz, Jason D. Myers, Jasbinder S. Sanghera.
Application Number | 20190341516 16/512450 |
Document ID | / |
Family ID | 54355837 |
Filed Date | 2019-11-07 |
United States Patent
Application |
20190341516 |
Kind Code |
A1 |
Frantz; Jesse A. ; et
al. |
November 7, 2019 |
MICROSTRUCTURED ZnO COATINGS FOR IMPROVED PERFORMANCE IN Cu(In,
Ga)Se2 PHOTOVOLTAIC DEVICES
Abstract
A microstructured ZnO coating that improves the performance of
Cu(In,Ga)Se.sub.2 (CIGS) photovoltaic (PV) devices via two
mechanisms; it acts an antireflective layer with superior
non-normal performance to thin film anti-reflective (AR) coatings,
and it scatters a large fraction of incoming light at a large
angle, resulting in absorption that is on average closer to the p-n
junction.
Inventors: |
Frantz; Jesse A.;
(Washington, DC) ; Myers; Jason D.; (Alexandria,
VA) ; Bekele; Robel Y.; (Washington, DC) ;
Sanghera; Jasbinder S.; (Ashburn, VA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The Government of the United States of America, as represented by
the Secretary of the Navy |
Arlington |
VA |
US |
|
|
Family ID: |
54355837 |
Appl. No.: |
16/512450 |
Filed: |
July 16, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
14701857 |
May 1, 2015 |
|
|
|
16512450 |
|
|
|
|
61986940 |
May 1, 2014 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 31/02168 20130101;
H01L 31/0749 20130101; H01L 31/18 20130101; Y02E 10/541 20130101;
H01L 31/02366 20130101; H01L 31/0322 20130101 |
International
Class: |
H01L 31/0749 20060101
H01L031/0749; H01L 31/0216 20060101 H01L031/0216; H01L 31/0236
20060101 H01L031/0236; H01L 31/18 20060101 H01L031/18; H01L 31/032
20060101 H01L031/032 |
Claims
1. A photovoltaic device, comprising: a p-type layer on a
substrate; an n-type layer on the p-type layer, forming a p-n
junction; a layer of ZnO on the n-type layer; a layer of
aluminum-doped ZnO (AZO) on the ZnO; and a continuous
microstructured ZnO topmost layer on the AZO, wherein the
continuous microstructured ZnO topmost layer comprises
antireflective surface structures on the AZO layer and scatters
incoming light, increasing absorption of scattered light close to
the p-n junction.
2. The photovoltaic device of claim 1, wherein the substrate
comprises glass with a Mo bottom contact.
3. The photovoltaic device of claim 1, wherein the p-type layer is
about 2 .mu.m thick.
4. The photovoltaic device of claim 1, wherein the n-type layer is
about 50 .mu.m thick.
5. The photovoltaic device of claim 1, wherein the ZnO layer on the
n-type layer is about 60 nm thick.
6. The photovoltaic device of claim 1, wherein the AZO layer is
about 200 nm thick.
7. The photovoltaic device of claim 1, wherein the ZnO layer, the
AZO layer, and the continuous microstructured ZnO topmost layer are
deposited on the substrate at a substrate temperature of
200.degree. C.
8. The photovoltaic device of claim 1, wherein the antireflective
surface structures in the continuous microstructured ZnO topmost
layer have a peak-to-peak height of about 500 nm or less.
9. The photovoltaic device of claim 1, wherein the p-type layer
comprises Cu(In, Ga)Se.sub.2 (CIGS), CuInSe.sub.2 (CIS),
CuGaSe.sub.2 (CGS), Cu.sub.2ZnSn(S,Se).sub.4 (CZTS), CdTe,
amorphous Si, or any combination thereof.
10. The photovoltaic device of claim 1, where the n-type material
is CdS.
11. A photovoltaic device, made by the method comprising: coating a
substrate with a p-type layer; depositing an n-type layer on the
p-type layer, forming a p-n junction; depositing a layer of ZnO on
the n-type layer; depositing a layer of aluminum-doped ZnO (AZO) on
the ZnO; depositing a continuous top layer of ZnO on the AZO; and
etching the ZnO top layer to form a textured continuous ZnO topmost
layer comprising subwavelength surface structures; wherein the
textured continuous ZnO topmost layer comprises antireflective
surface structures on the AZO layer and scatters incoming light,
increasing absorption of scattered light close to the p-n
junction.
12. The photovoltaic device of claim 11, wherein the substrate
comprises glass with a Mo bottom contact.
13. The photovoltaic device of claim 11, wherein the p-type layer
is about 2 .mu.m thick.
14. The photovoltaic device of claim 11, wherein the n-type layer
is about 50 .mu.m thick.
15. The photovoltaic device of claim 11, wherein the ZnO layer on
the n-type layer is about 60 nm thick.
16. The photovoltaic device of claim 11, wherein the AZO layer is
about 200 nm thick.
17. The photovoltaic device of claim 11, wherein the ZnO layer, the
AZO layer, and the continuous ZnO top layer are deposited on the
substrate at a substrate temperature of 200.degree. C.
18. The photovoltaic device of claim 1, wherein the antireflective
surface structures in the textured continuous ZnO topmost layer
have a peak-to-peak height of about 500 nm or less.
19. The photovoltaic device of claim 1, wherein the p-type layer
comprises Cu(In, Ga)Se.sub.2 (CIGS), CuInSe.sub.2 (CIS),
CuGaSe.sub.2 (CGS), Cu.sub.2ZnSn(S,Se).sub.4 (CZTS), CdTe,
amorphous Si, or any combination thereof.
20. The photovoltaic device of claim 1, where the n-type material
is CdS.
Description
PRIORITY CLAIM
[0001] The present application is a divisional application of U.S.
application Ser. No. 14/701,857, filed on May 1, 2015 by Jesse A.
Frantz et al., entitled "Microstructured ZnO Coatings for Improved
Performance in Cu(In,Ga)Se.sub.2 Photovoltaic Devices," which
claimed the benefit of U.S. Provisional Application No. 61/986,940,
filed on May 1, 2014 by Jesse A. Frantz et al., entitled
"Microstructured ZnO Coatings for Improved Performance in
Cu(In,Ga)Se.sub.2 Photovoltaic Devices," the entire contents of
both are incorporated herein by reference.
BACKGROUND OF THE INVENTION
Field of the Invention
[0002] The present invention relates to a microstructured ZnO
coating that improves the performance of Cu(In,Ga)Se.sub.2 (CIGS)
photovoltaic (PV) devices.
Description of the Prior Art
[0003] CIGS thin film PV devices typically employ a transparent
conductive oxide film, most commonly tin-doped indium oxide (ITO)
or aluminum-doped zinc oxide (AZO), as a top electrode. Both ITO
and AZO have refractive indices of around 2 at a wavelength of 500
nm, resulting in Fresnel reflections with a magnitude of
approximately 11% at normal incidence. In uncoated CIGS films,
therefore, a significant fraction of the incoming light is lost to
reflection.
[0004] In high-performance CIGS devices, an anti-reflective (AR)
coating, most often a quarter-wave of MgF.sub.2, is employed to
reduce surface reflections. This coating results in an improvement
in performance at normal incidence associated with an increase in
the short circuit current density, J.sub.SC, of approximately 5%.
Repins et al., "Required material properties for high-efficiency
CIGS modules," SPIE 7409, Thin Film Solar Technology, 74090M
(2009). While single-layer AR coatings may exhibit excellent
performance for a particular wavelength at a fixed incident angle,
performance typically suffers away from the design wavelength and
incident angle. Dobrowolski et al., "Toward perfect antireflection
coatings: numerical investigation," Appl. Opt., 41, 16, 3075-83
(2002). Since CIGS devices are often used in applications without
tracking and in environments with significant scattered light, the
performance at non-normal incidence is important. A coating that
improves performance across both wide spectral and angular ranges
is therefore desirable.
[0005] To best match the dispersion of the existing device surface,
one approach is to create a structured layer of the same material
present at the device/air interface, i.e. the top contact material.
Such an anti-reflective surface structure (ARSS), whether its
structure is ordered or random, can achieve high AR performance
across a broad spectral and angular range. Florea et al., "Recent
advancements in anti-reflective surface structures (ARSS) for near-
to mid-infrared optics," SPIE 8708, Window and Dome Technologies
and Materials XIII, 87080P (2013). For instance, ZnO nanorods,
grown with an aqueous process, have been shown to decrease the
surface reflection at normal incidence when grown on CIGS devices.
Shin et al., "Bottom-up grown ZnO nanorods for an antireflective
moth-eye structure on CuInGaSe.sub.2 solar cells," Sol. Energy
Mater. Sol. Cells, 95, 9, 2650-2654 (2011).
[0006] A second consideration for light collection in CIGS PV
devices is the proximity to the p-n junction of photon absorption.
In a typical CIGS device a p-type CIGS layer (typically about 2
.mu.m thick) is coated with an n-type material such as CdS in order
to form a p-n junction. A significant portion of the incident light
is absorbed in the CIGS far (>500 nm) away from the p-n
junction. If light is deflected at a large angle away from the
surface normal, a larger percentage of light is absorbed close to
the p-n junction, resulting in less recombination and ultimately
higher efficiency.
[0007] An effect that has been observed in ZnO is surface texturing
during a wet etch in HCl. A previous approach has been applied to
fabricate textured bottom contacts for a-Si solar cells. Kluth et
al., "Texture etched ZnO:Al coated glass substrates for silicon
based thin film solar cells," Thin Solid Films, 351, 247-253
(1999).
BRIEF SUMMARY OF THE INVENTION
[0008] The present invention provides a microstructured ZnO coating
that improves the performance of Cu(In,Ga)Se.sub.2 (CIGS)
photovoltaic (PV) devices via two mechanisms; it acts as an
antireflective layer with superior non-normal performance to thin
film anti-reflective (AR) coatings, and it scatters a large
fraction of incoming light at a large angle, resulting in
absorption that is on average closer to the p-n junction.
[0009] The performance of thin film Cu(In,Ga)Se.sub.2 (CIGS)
photovoltaics is typically degraded by light lost due to the high
reflectivity of the transparent top contact and by recombination
resulting from carrier generation far from the junction.
Traditional antireflective (AR) coatings are insufficient to
address the former issue, particularly at non-normal incidence. The
present invention provides a novel microstructured ZnO coating that
acts as an antireflective layer and scatters a large fraction of
the incoming radiation at a large angle, resulting in absorption
that is closer to the junction. This coating, formed via a wet etch
process, results in performance comparable to that of uncoated
films at normal incidence and an increase of up to 25% in the short
circuit current and 18% in device efficiency at non-normal
incidence.
[0010] The present invention has many advantages. The patterned ZnO
acts an antireflective layer with superior non-normal performance
to thin film AR coatings, improving CIGS PV device conversion
efficiency. The patterned ZnO results in scattering of a large
fraction of the incoming light at a large angle, resulting in
absorption that is on average closer to the junction, improving
CIGS PV device conversion efficiency. The ZnO coating is compatible
with existing CIGS processing. A large scattering angle may permit
thinner CIGS layers to be used in PV devices, resulting in less
material usage.
[0011] These and other features and advantages of the invention, as
well as the invention itself, will become better understood by
reference to the following detailed description, appended claims,
and accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a schematic diagram of a CIGS device with a ZnO
ARSS coating.
[0013] FIG. 2 shows SEM cross sections (top) and images taken at
20.degree. from normal incidence of ZnO ARSS structures obtained
with etch times varying from 0-30 s in 0.5% HCl solution.
[0014] FIG. 3 shows the peak-to-peak height of ARSS features,
measured as a function of etch time.
[0015] FIG. 4 shows the change in J.sub.SC, compared before and
after ARSS deposition, as a function of illumination angle.
DETAILED DESCRIPTION OF THE INVENTION
[0016] The present invention provides a new product that includes
antireflective surface structures (ARSS) formed by chemical etching
ZnO formed on top of CIGS devices. The ZnO structures improve PV
device performance through a combination of two effects, the
antireflective (AR) properties of the ARSS and the improvement
resulting from large-angle scattering.
[0017] In one embodiment as depicted in FIG. 1, a soda lime glass
(SLG) substrate 10 with a sputtered Mo bottom contact 12 was coated
with 2 .mu.m of CIGS 14 via single-step evaporation. Approximately
50 nm of CdS 16 was then deposited by chemical bath deposition. For
the top contact, 60 nm of ZnO 18 was deposited by reactive
sputtering with flowing O.sub.2 in order to increase resistivity
and transparency, and this was followed by a conductive 200 nm
thick layer of aluminum-doped ZnO (AZO) 20 deposited by sputtering.
Both ZnO and AZO were deposited at a substrate temperature of
200.degree. C. Ni/Al grids, composed of 60 nm of Ni followed by 400
nm of aluminum, were deposited via electron beam evaporation.
Samples were scribed by hand to obtain individual cells, each with
an area of approximately 0.5 cm.sup.2. PV devices were then
characterized prior to ARSS coating.
[0018] The contact pads for the grids 24 were protected with
photoresist, and ZnO ARSS 22 were formed on top of the devices. An
870 nm thick layer of ZnO was sputtered on top of the complete CIGS
device, again with flowing O.sub.2 and at a substrate temperature
of 200.degree. C. This layer was chemically etched in a 0.5% HCl
solution at room temperature for 0-30 s resulting in a textured ZnO
surface. The photoresist protecting the contact pads was removed,
and devices were characterized.
[0019] Samples for cross sectional SEM analysis were obtained by
mechanically breaking samples. Light J-V curves were obtained in a
solar simulator under one sun, AM 1.5 G illumination calibrated
using a Si reference cell. The setup was configured to allow for
angular measurements of up to 60.degree. from normal incidence.
Dark current measurements were obtained with a Keithley 2400
SourceMeter in a darkened enclosure in order to evaluate diode
properties of the devices.
[0020] Samples consisting of a ZnO/AZO electrode with ARSS coatings
were etched for times varying from 0-30 s in dilute HCl. Samples
were visibly hazy in transmission after etching. Spectroscopic
measurements of etched ZnO films deposited on glass substrates
showed an absolute decrease of .about.5% from 350-1200 nm in
specular reflection.
[0021] SEM images showing cross sections and images taken at
20.degree. from normal incidence are shown in FIG. 2. Prior to
etching, the ZnO exhibited a small amount of surface roughness that
increased rapidly with etch time. Features were subwavelength and
consistent across the etched surface. For the 20 and 30 s etches,
some features extended through the entire ZnO film but not the
underlying electrode, indicating that the etch rate of AZO is
smaller than that of ZnO. This is fortuitous in that the AZO layer
acts as a barrier, preventing the HCl etch from damaging other
layers of the device. This result was consistent with dark current
measurements, made before and after ARSS deposition, showing that
the diode properties of the junction were preserved.
[0022] The depth of ARSS features, measured peak-to-peak from
cross-sectional SEM images, is shown in FIG. 3. Feature height
initially increased with etch time, peaks at approximately 500 nm
for an etch time of 15 s, and decreased gradually with further
etching. The decrease in thickness resulted from etching of the
tallest features while no ZnO remained to etch on the bottom.
Feature height could potentially be increased further by using a
thicker ZnO film.
[0023] Light J-V measurements were obtained for films with varying
etch times for angles ranging from 0-60.degree.. The open circuit
voltage, V.sub.OC, and fill factor (FF) were found to decrease
slightly, by <10%, for all etch times and angles. This was
attributes to the extra anneal that occurred during ZnO deposition.
Further optimization of ZnO deposition parameters is expected to
reduce this effect. The most pronounced change, however, was a
dramatic increase in J.sub.SC. FIG. 4 shows J.sub.SC as a function
of incident angle for varying etch times. A slight increase of
approximately 5% was evident for the un-etched sample for all
angles--potentially resulting from the extra anneal or from
scattering caused by the intrinsic texture of the un-etched ZnO
surface. The etched samples each exhibited a J.sub.SC increase of
approximately 10% at normal incidence that further increased with
incident angle. The J.sub.SC of the 20 s sample increased most--by
14% at 30.degree. and 25% at 60.degree..
[0024] As a result of the increase in J.sub.SC of the 20 s sample,
its performance improved the most of all devices. Table 1 shows PV
parameters for this sample. While the efficiency, increased only
slightly, from 10.4% to 10.5% at normal incidence. It increased
more significantly for non-normal incidence with a relative
improvement of approximately 18% for 60.degree. illumination. This
is consistent with decreased surface reflection.
TABLE-US-00001 TABLE 1 Device Results for 20 s Etched Sample
Condition V.sub.OC (mV) J.sub.SC (mA/cm.sup.2) FF (%) .eta. (%)
0.degree. Before 520.3 30.9 64.6 10.4 0.degree. After 514.7 33.8
60.1 10.5 30.degree. Before 517.9 25.5 65.7 8.7 30.degree. After
512.8 29.0 60.8 9.0 60.degree. Before 503.5 13.3 66.7 4.5
60.degree. After 503.5 16.5 64.0 5.3
[0025] It is significant to note that the increase in J.sub.SC at
60.degree. is greater than the 17% Fresnel reflection expected at
this angle. Thus, the AR properties of the ARSS alone are
insufficient to explain the increased current. It is clear that the
scattering properties of the coating, resulting in absorption
closer to the junction, are necessary to fully explain the increase
in J.sub.SC at large angles.
[0026] The PV absorber could be a different thin film PV absorber,
such as CuInSe.sub.2 (CIS), CuGaSe.sub.2 (CGS),
Cu.sub.2ZnSn(S,Se).sub.4 (CZTS), CdTe, amorphous Si, or
organics.
[0027] A mask could be deposited on the ZnO prior to etching in
order to affect the layer's post-etching morphology.
[0028] Deposition parameters for the ZnO film such as substrate
temperature, partial pressure, and deposition power could be
adjusted in order to affect the layer's post-etching
morphology.
[0029] The oxygen content of the ZnO film could be varied by
adjusting target composition or O.sub.2 flow during deposition in
order to affect the layer's post-etching morphology.
[0030] The ZnO film could doped with an agent that affects grain
formation--resulting in changes in grain size, shape or
orientation--in order to affect the layer's post-etching
morphology.
[0031] The above descriptions are those of the preferred
embodiments of the invention. Various modifications and variations
are possible in light of the above teachings without departing from
the spirit and broader aspects of the invention. It is therefore to
be understood that the claimed invention may be practiced otherwise
than as specifically described. Any references to claim elements in
the singular, for example, using the articles "a," "an," "the," or
"said," is not to be construed as limiting the element to the
singular.
* * * * *